1. Field of the Invention
Embodiments of the present invention relate to a display device and, more particularly, to a transflective liquid crystal display (LCD) device and a fabrication method thereof. Although embodiments of invention are suitable for a wide scope of applications, it is particularly suitable for simplifying a fabrication process and improving production yield by reducing the number of masks and also suitable for implementing high picture quality by preventing generation of wavy noise.
2. Description of the Related Art
As the consumer's interest in information displays is growing and the demand for portable (mobile) information devices is increasing, research and commercialization of light and thin flat panel displays (“FPD”) has increased.
Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images by using optical anisotropy of liquid crystal. LCD devices exhibit excellent resolution and color and picture quality, so it is widely used for notebook computers or desktop monitors, and the like.
The LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal molecules in a pixel part are driven by using amorphous silicon thin film transistors (a-Si TFTs) as switching elements.
In the fabricating process of the LCD, a plurality of masking processes (namely, photographing processes) are performed to fabricate the array substrate including the TFTs, so a method for reducing the number of masking process will increase productivity.
The general structure of the LCD will now be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing a general LCD.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes gate lines 16 and data lines 17 which are arranged vertically and horizontally to define a plurality of pixel regions (P), TFTs, switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel regions (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrates 5 and the array substrate 10 is made by an attachment key formed on the color filter substrate 5 or the array substrate 10.
The general LCD expresses an image by light emitted from a light source such as a backlight positioned at a lower portion of a liquid crystal panel. However, the actual amount of light transmitted through the liquid crystal panel is about 7% of the light generated by the backlight, causing severe loss of light, so power consumption by the backlight is high.
Recently, to solve the problem of the high power consumption, a reflective LCD that does not use such a backlight has been studied. The transflective LCD uses natural light as a means for expressing an image, without such power consumption caused by the backlight, so it can be used in a carried-around state for a long time.
Unlike an existing transmissive LCD, the reflective LCD uses an opaque material having reflectivity characteristics at a pixel region to reflect light made incident from an external source to thus express an image.
However, because natural or an artificial light source does not exist always, the reflective LCD can be used during day time when natural light is present or in an office or in a building where an external artificial optical source is provided. Namely, the reflective LCD cannot be used in a dark environment in which natural light is not present.
To solve the problem, a transflective LCD, which combines the advantages of the reflective LCD using natural light and the transmissive LCD that uses a backlight, is being actively studied. The transflective LCD can be changed to a reflective mode and a transmissive mode according to a user intention, and light of the backlight, an external natural light source or an artificial light source can be all used, so power consumption can be reduced without being limited to the surrounding environments.
FIGS. 2A to 2F are cross-sectional views sequentially showing a fabrication process of an array substrate of the general transflective LCD.
As shown in FIG. 2A, a gate electrode 21 made of a conductive material is formed by using a photolithography process (a first masking process) on a substrate.
Next, as shown in 2B, a first insulation film 15a, an amorphous silicon thin film and an n+ amorphous silicon thin film are sequentially deposited over the entire surface of the substrate 10 with the gate electrode 21 formed thereon, and the amorphous silicon thin film and the n+ amorphous silicon thin film are selectively patterned by using the photolithography process (a second masking process) to form an active pattern 24 formed of the amorphous silicon thin film on the gate electrode 21.
In this case, the n+ amorphous silicon thin film pattern 25 which has been patterned in the same form as the active pattern 24 is formed on the active pattern 24.
Thereafter, as shown in FIG. 2C, a conductive metal material is deposited over the entire surface of the array substrate 10 and then selectively patterned by using the photolithography process (a third masking process) to form a source electrode 22 and a drain electrode 23 at an upper portion of the active pattern 24. At this time, a certain portion of the n+ amorphous silicon thin film pattern formed on the active pattern 24 is removed through the third masking process to form an ohmic-contact layer 25′ between the active pattern 24 and the source and drain electrodes 22 and 23.
Subsequently, as shown in FIG. 2D, a second insulation film 15b, namely, an organic insulation film such as acryl, is deposited over the entire surface of the array substrate 10 with the source electrode 22 and the drain electrode 23 formed thereon, and a portion of the second insulation film 15b is removed through the photolithography process (a fourth masking process) to form a contact hole 40 exposing a portion of the drain electrode 23.
In this case, as shown, the surface of the second insulation film 15b is formed to be irregular (i.e., uneven, rough, jagged, bumpy, undulated, wavy, rippled, furrowed, ruffed, indented, serrated, etc.) to enhance reflection efficiency in the reflective mode.
As shown in FIG. 2E, a conductive metal material having good reflectivity is deposited over the entire surface of the array substrate 10 with the second insulation film 15b formed thereon, and then selectively patterned by using the photolithography process (a fifth making process) to form a reflective electrode 18b electrically connected with the drain electrode 23 via the contact hole 40.
As shown in FIG. 2F, a transparent conductive metal material is deposited over the entire surface of the array substrate 10, and then, a pixel electrode 18a is formed over the entirety of the pixel region including a reflective part where the reflective electrode 18b has been formed, by using a photolithography process (a sixth masking process).
As mentioned above, in fabricating the array substrate including TFTs of the general transflective LCD, a total of six photolithography processes are necessarily performed. That is, the general transflective LCD requires more photolithography processes compared to that of the transmissive LCD.
The photolithography process is a process of transferring a pattern formed on a mask onto the substrate on which a thin film is deposited to form a desired pattern, which includes a plurality of processes such as a process of coating a photosensitive solution, an exposing process and a developing process, etc., which, thus, degrades a production yield.
In particular, because the masks designed for forming the pattern are quite expensive, as the number of masks used in the processes increases, the fabrication cost of the LCD increases proportionally.
A technique for fabricating the array substrate by performing the masking process four times by forming the active pattern and the source and drain electrodes using a single masking process having a slit (diffraction) mask has been proposed.
However, because the active pattern, the source and drain electrodes and the data lines are simultaneously patterned by performing an etching process twice with the slit mask, the active pattern protrusively remains near the lower portions of the source electrode, the drain electrode and the data lines.
The protrusively remaining active pattern is formed of an intrinsic amorphous silicon thin film, so the protrusively remaining active pattern is exposed to light from the lower backlight, generating an optical current. The amorphous silicon thin film reacts slightly to a blinking of the light from the back light, and repeatedly becomes activated and deactivated, which causes a change in the optical current. The changing optical current component is coupled with a signal flowing in the neighboring pixel electrodes so as to distort movement of the liquid crystal molecules positioned at the pixel electrodes. As a result, a wavy noise is generated such that a wavy fine line appears on a screen of the LCD.
In addition, because the active pattern positioned at the lower portion of the data lines has portions that protrude at a certain height from both sides of the data lines, the opening region of the pixel part is encroached by as much as the protrusion height, thus resulting in a reduction in an aperture ratio of the LCD.